A light-emitting device for emitting white color, constituted by a combination of a gallium nitride (GaN)-type blue-emitting diode as a semiconductor light-emitting element and a phosphor as a wavelength-conversion material, has a long useful life with small power consumption and thus has attracted an attention as a light-emitting source for an image display device or a lighting system.
With such a light-emitting device, the phosphor used therein absorbs visible light in a blue color region emitted from the GaN-type blue-emitting diode and emits yellow light, whereby white color emission is obtainable by the color mixing of the yellow light with the blue light of the light-emitting diode not absorbed by the phosphor. As such a phosphor, typically, a phosphor is known which comprises, as matrix, an yttrium/aluminum composite oxide (Y3Al5O12) and which contains cerium (Ce) as a luminescent center ion in the matrix. However, this phosphor is difficult to prepare, for example, as the is firing temperature is high, and it was not satisfactory also from the viewpoint of the temperature quenching.
As a substitute yellow phosphor, the present inventors have invented a phosphor having a basic structure being Ca3Sc2Si3O12:Ce3+ (hereinafter referred to simply as “CSS phosphor”) and previously filed a patent application (Patent Document 1). Namely, it is a phosphor which comprises, as matrix, a compound of a garnet crystal structure represented by the following formula and which contains a luminescent center ion in the matrix.M1′a′M2′b′M3′c′Od′wherein M1′ is a bivalent metal element, M2′ is a trivalent metal element, M3′ is a tetravalent metal element, a′ is a number within a range of from 2.7 to 3.3, b′ is a number within a range of from 1.8 to 2.2, c′ is a number within a range of from 2.7 to 3.3, and d′ is a number within a range of from 11.0 to 13.0.
The Patent Document 1 discloses Ca as the bivalent metal element M1′ and also discloses a phosphor wherein a part of Ca is substituted by e.g. Mg or Zn. Such CSS phosphor is one which emits from blue- to yellow-color depending upon the composition and is a high performance phosphor which can be used also as a green phosphor for a light-emitting device having a GaN-type blue-emitting diode, a green phosphor and a red phosphor combined.
Patent Document 1: JP-A-2003-64358